1. Field of the Invention
The invention relates to a process for producing coarse-particle, monodisperse anionic-exchanger gels with high stability and purity.
2. Brief Description of the Prior Art
In recent times, increasing importance has been attached to anion exchangers whose particle size is as large and as uniform as possible, the particle size preferably being >800 μm, since the coarse grain size of the exchanger bed gives economic advantages in many applications. These advantages are provided in particular by monodisperse ion exchangers. They can be obtained by functionalizing monodisperse bead polymers articularly coarse-particle materials are suitable for use in gas adsorption, catalysis and hydrometallurgy. Processes usually used hitherto can produce ion exchangers with grain size from 300 to 800 μm. While the literature describes a number of methods for producing ion exchangers with particularly coarse grain size >800 μm, i.e. relatively coarse exchangers, these methods have associated disadvantages described below.
U.S. Pat. No. 2,992,544 describes the copolymerization of chloromethylstyrene and divinyl-benzene and the conversion to the anion exchanger. U.S. Pat. No. 3,122,514 describes the synthesis of coarse ion exchangers from polyvinyl compounds and prepolymers. However, both synthetic routes utilize starting materials which are not available in sufficient quantity and quality.
Another synthetic route gives ion exchangers composed of fine ion-exchanger grains with diameter mostly below 100 μm, or of broken ion-exchanger material, which are embedded within an inert material. Olefin polymers are often used as a means of embedding, as described in German Patent Specifications 1 285 170, 2 237 953 and 2 343 417, and also in DD Patent 114 350. A substantial disadvantage of these materials, however, is the low utilizable capacity resulting from the inert polymer content, and the difficulty of accessing the ion-exchanging particles.
Similarly DD Patent 158905 describes particles which are not always suitable for industrial applications, which are prepared by a feed procedure on inert polymer mouldings mainly based on olefin polymers, using styrene-divinylbenzene monomers, with subsequent modification to give ion-exchanging materials.
Another route to the production of coarse-particle ion exchangers consists in the seed-feed polymerization method. The underlying principle consists in preparing a seed from styrene and divinylbenzene with a low level of crosslinking. The crosslinker content here is to be from 0.05 to 0.6%. The use of the starting polymer with a very low level of crosslinking introduces more extractable constituents into the synthesis or results in more extractable constituents being leached out of the finished exchanger. Feed polymerization then takes place, using from 2 to 15 times the weight of the seed polymer, with is a great variation in the crosslinker content (from 0.5 to 50% by weight). EP-A 0 062 088 describes this principle, including conversion of the chloromethylated intermediate product to give the anion exchanger, under gentle conditions in the presence of dimethoxymethane.
DD Patent 229 696 improves the stability of the final product in the feed procedure by adding a means of inertization.
DD Patent 158 906 describes the synthesis of coarse-particle polymers as precursors for corresponding ion exchangers. Here again, a means of inertization is added to improve mechanical stability, and a specific stirrer is used to give coarse particles.
EP-A-0 098 130 and EP-A-0 101 943 describe the seed-feed process for preparing monodisperse bead polymers, which involves swelling a monodisperse polymer (“seed”) in the monomer, which is then polymerized.
The seed polymers used in seed-feed processes should have a high swelling index so that during the seed-feed process they can absorb a large amount of the added monomers. The swelling index (SI) is defined as the quotient calculated from the volume of the swollen polymer and the volume of the non-swollen polymer. The swelling index can be controlled in a known manner via the content of crosslinker: low crosslinker contents lead to high swelling indices and vice versa. For example, styrene polymers crosslinked using from 0.8 to 2.0% by weight of divinylbenzene have swelling indices of from 8 to 2.5 in toluene. However, seed polymers with a very low level of crosslinking have a very high fraction of uncrosslinked extractable constituents. This content of extractable constituents in the seed polymer is in many respects undesirable, because                1. polymer fractions dissolved out from the seed by the monomer added cause the particles to adhere to one another, and this can disrupt the polymerization of the swollen seed;        2. the concentration of the dissolved-out fraction increases in the reaction solutions used for the functionalization, and this can make the functionalization to prepare the anion exchangers more difficult;        3. the anion exchangers may comprise increased amounts of soluble polymers, and this can lead to undesired leaching from the ion exchangers.        
Another problem with the known anion exchangers is that their mechanical and osmotic stability is not always adequate. This becomes particularly clear when exchangers with coarse grain sizes are synthesized. For example, the mechanical or osmotic forces which arise can fracture anion exchanger beads during their production or during their use. A rule applicable to all applications of anion ex-changers is that the exchangers must retain their bead form, and no degradation, partial or complete, or breakdown into fragments must occur during application. Fragments can pass into the solutions to be treated during application and contaminate these very solutions or render separation processes more difficult, and can lead to losses of adsorbed material. In addition, the presence of damaged anion exchangers is disadvantageous for the actual functioning of the ion exchangers used in column processes. Fragmented materials cause an increased pressure loss in the column system and thus reduce the throughput of the liquid to be purified through the column, or make separation more difficult in batch process applications.
The processes described above indicate clearly the problems associated with the synthesis of coarse-particle monodisperse ion exchangers which give rise to final products with coarse particle size and desirable physicochemical properties.